If you wish to remove Metadrive, you just have to remove the permissions you previously gave to our app in this link: https://myaccount.google.com/permissions
Once you clicked there, the following window will appear. There, you must click on Metadrive. 
Once you clicked on Metadrive, a blue button appears with the text «Remove Access», as shown in the picture below. If you click on it, you will see a pop-up to confirm if you want to remove the access you gave to Metadrive. There, you click «Ok».
Your permissions have already been removed!